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ME 2134E Lab Report

Motor Characteristics

LIN SHAO DUN A0066078X

YEE KOK HENG A0066142M

LUM SOON HENG A0066070M

Lab Group 2B

Date 18th Oct 2011

1

TABLE OF CONTENTS

OBJECTIVES 2

EX P E R I M E N T RE S U L T 2

DISCUSSION 7

CONLCUSION 12

2

OBJECTIVES

An electric motor converts electrical energy into mechanical energy. Electric motors are found in

applications as diverse as industrial fans, blowers and pumps, machine tools, household

appliances, power tools, and disk drives. They may be powered by D.C (direct current), i.e. a

battery powered portable device or motor vehicle, or by A.C (alternating current) from a central

electrical distribution grid or inverter.

Study of electric motor’s characteristics will enable us to have a better understanding of motor’s

performance curve as well as correctly size a motor for certain purpose.

The objectives of this experiment include the following:

1) To be familiar with the wiring and basic characteristics of the following motors:

D.C Series Motor

D.C Shunt Motor

A.C 3-Phase Squirrel Cage Induction Motor

2) To examine the relationship between Torque, Speed, Voltage, and Current for various types

of motor connections in no-load and loaded configurations.

3) To be familiar with the usage of common meters such as multimeter and tachometers.

EXPERIMENT RESULT

1. D.C Series Motor: Constant-Load Test

Table 1: DC Series Motor Constant-Load Test results

Volts (V) Speed (rpm) Current (A)

180.3 2300 0.451

159.8 2056 0.436

140.4 1850 0.425

119.4 1578 0.417

99.9 1313 0.417

79.8 1033 0.410

59.5 725 0.410

3

Chart 1: DC Series Motor Constant- Load Test result

2. D.C Series Moto Load Test

Table 2: D.C Series Motor Load Test Results

Volts (V) Speed (rpm) Torque (N.m) Current (A)

180.1

2220 0.1 0.450

1895 0.2 0.541

1680 0.3 0.620

1520 0.4 0.694

1409 0.5 0.758

1305 0.6 0.832

1226 0.7 0.892

1160 0.8 0.952

1106 0.9 1.009

1056 1.0 1.071

0.40

0.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

0.49

0.50

0

20

40

60

80

100

120

140

160

180

200

700 900 1100 1300 1500 1700 1900 2100 2300

Cu

rren

t (A

)

Vo

lts

(V)

Speed (rpm)

Current and Voltage against Speed - Constant Load

Voltage Against Speed

Current Against Speed

4

Chart 2: DC Series Motor Load Test result

3. DC Shunt Motor No Load Test Results

Table 3: DC Shunt Motor No Load Test Results

Volts (V) Speed (rpm) Field Current (A) Line Current (A)

240.5 1500 0.171 0.357

220.5 1445 0.156 0.339

199.8 1379 0.141 0.330

180.3 1320 0.128 0.323

159.9 1257 0.113 0.316

140.1 1194 0.099 0.314

119.3 1120 0.085 0.318

99.8 1043 0.071 0.329

79.5 944 0.057 0.346

60.4 806 0.043 0.380

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1000 1200 1400 1600 1800 2000 2200

Cu

rren

t (A

)

To

rqu

e (N

.m)

Speed (rpm)

Current and Torque against Speed - Loaded

Torque against Speed

Current against Speed

5

Chart 3: DC Shunt Motor No Load Test result

4. DC Shunt Motor Load Test Results

Table 4: DC Shunt Motor Load Test Results

Volts (V) Speed (rpm) Torque (N.m) Field Current (A) Line Current (A)

240.5

1487 0.1 0.175 0.382

1460 0.2 0.173 0.462

1424 0.3 0.172 0.530

1400 0.4 0.172 0.608

1378 0.5 0.171 0.687

1348 0.6 0.170 0.744

1328 0.7 0.169 0.831

1312 0.8 0.168 0.887

1276 0.9 0.167 0.987

1266 1.0 0.167 1.060

0.00

0.04

0.08

0.12

0.16

0.20

0

50

100

150

200

250

800 900 1000 1100 1200 1300 1400 1500

Fie

ld C

urr

ent

(A)

Vo

lts

(V)

Speed (rpm)

Field Current, Voltage against Speed

Voltage Against Speed

Field Current vs. Speed

6

Chart 4: DC Shunt Motor Load Test result

5. AC 3-Phase Motor 400V AC Result

Table 5: AC 3-Phase Motor 400V AC Result

Volts (V) Torque (N.m) Speed (rpm) Line Current (A)

400

0 1465 0.24

0.1 1460 0.24

0.2 1459 0.25

0.3 1446 0.26

0.4 1437 0.27

0.5 1427 0.29

0.6 1415 0.31

0.7 1402 0.33

0.8 1381 0.35

0.9 1374 0.37

1.0 1362 0.40

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1250 1300 1350 1400 1450 1500

Lin

e C

urr

en

t (A

)

To

rqu

e (N

.m)

Speed (rpm)

Torque, Line Current against Speed

Torque Against Speed

Line Current Against Speed

7

Chart 5: AC 3-Phase Motor 400V AC result

DISCUSSION

1. Questions from Experiments on DC Series Motor

1.1. Why must the DC series motor be always started under load and give

examples in your answer?

The speed of a series motor with no load connected to it increases rapidly to the point where

the motor may become damaged. Usually, either the bearings are damaged or the windings

fly out of the slots in the armature. Some load must always be connected to a series motor

before turn it on. This precaution is primarily for large motors. Small motors, such as those

used in electric hand drills, have enough internal friction to load themselves.

For a series motor,

Where – Back emf of armature

– Rotational speed of armature

– Voltage supplies to motor

– Current in armature

– Resistance of the armature

–Motor constant

–Flux

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0

0.2

0.4

0.6

0.8

1

1.2

1360 1380 1400 1420 1440 1460

Lin

e C

urr

en

t (A

)

To

rqu

e (N

.m)

Speed (rpm)

Torque, Line Current against Speed

"Torque Against Speed

Series2

𝜔 ↑ → 𝑏𝑎𝑐𝑘 𝑒𝑚𝑓 ↑ → 𝐼𝐹 ↓ → 𝜙 ↓

8

From above equation we can see in DC motor the speed is inversely proportion to the flux.

When a DC series motor starts without load the speed will increase, as speed increases the

back emf also increases which reduces the current flow through the series winding and

causes flux decreases and speed will increase further. Eventually the motor will accelerate to

a very high speed, which might damage the bearing and other components. When start the

motor with load, it is actually reducing the starting speed hence the motor will run safely.

A series motor works extremely well in applications require high torque and low speed for

starting and high speed and low torque for running as in the case of starting and moving

trains, household appliance like washing machine and fans.

1.2. Why is the current proportional to torque for a DC series motor?

For DC motor, torque is given by . Since , we have

Because in DC series motor, we have

This equation shows Torque is proportional to the square of current.

Below chart shows the relationship of current vs. torque base on experimental data.

1.3. Try your best to briefly explain the shape of the graphs obtained in this

experiment.

a) For the Constant- Load graph, the voltage is linearly proportional to the motor speed.

As we know

→ , in this equation:

only changes in small range ( 0.41~0.45) , , and are motor constant.

Hence this is a linear equation:

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

To

rqu

e (N

.m)

Current (A)

Current vs. Torque

Fit curve 𝑦 0.86𝑥

Experiment data

9

b) For the load graph, the shape of the Torque vs. Speed is a curve. This can be explained

from the equation of torque :

This is a 2nd

order equation; hence the graph is a parabolic.

2. Questions from Experiments on DC Shunt Motor

2.1. How the speeds is regulated in a DC shunt motor and give examples?

In a DC Shunt Motor, when the terminal voltage increases, the speed will also increase but

speed will decrease if the resistance of the armature circuit or the flux per

pole increases.

From equation:

If increases, will increase but if increases or increases, will decrease. Thus, the

three variables, namely the voltage, the coil resistance and the flux are often used to regulate

the speed of the motor, however, the most commonly way to control the speed is to vary the

voltage supply.

Examples of a DC shunt motors are the propellers of a model aircraft, whereby the speed of

the propellers still remain the speed even if the load (i.e. the wind resistance) increases.

Another example is the film projector used in cinemas, whereby the wheel of the film still

rotates at same speed despite changing loads (i.e. weight of the film cartridge as it turns from

one wheel to the other).

2.2. From no-load to full-load explain in your own words why there is little

speed variation over this range.

For Shunt motor, we have:

Re-arrange, we have:

For no-load to full-load, the speed will be approximately constant. This is because is

small and thus ⁄ term will be even smaller. Therefore ⁄ , which

means the speed only changes a little over the range.

10

When the motor switches from a no-load to a loaded condition, the motor begin to slower

down. Since a voltage is constant across the field, the field independent of variations in the

armature circuit. Once the load is applied, it will result in the reduction in speed which is

proportional to the back emf. To maintain the original voltage, the net voltage will increase

To increase the net voltage, the armature will draw more current and the armature current

will increase. That will result in the torque to increase. Increase in the torque causes the

motor armature to speed up. When the armature speed increase, the back emf will increase

and the armature current decrease. Finally it will eventually equalize and stop changing when

the torque reaches a level that requires turning the larger load.

In each case, all of this happens so rapidly that any actual change in speed is slight. There is

instantaneous tendency to change rather than a large fluctuation in speed. Therefore, there is

a little speed variation over the range

2.3. Try your best to briefly explain the Torque vs. Speed graph obtained in this

experiment.

For shunt motor, we have

Which means the Torque is a linear function of speed.

From the diagram we can see that increasing the load decreases the speed linearly. It shows

that while the armature current is dependent on the load, the field current is independent of

the load conditions. With the load applied, motor speed decreases and draws more current to

increase torque as shunt motor's torque is directly proportional to armature current. And we

also need to consider the Rotational Losses of a DC motor, includes all speed dependent

losses, such as bearings and brushes friction losses, windage losses, eddy current and

hysteretic losses in the armature core to maintain the speed variation. These losses are

independent of the load. The other losses are due to the resistance of the windings. Some

depend on the load (copper losses in the armature), others on the applied voltage (copper

losses in the shunt field winding).

Therefore, we can say that the speed of the shunt motor stays fairly constant throughout its

load range. If the variations are within an appropriate range, constant speed can be

maintained from no load to rated load.

11

3. Observations from Experiments on AC Three-Phase Induction Motor

From the laboratory results, the observations of the AC 3-phase induction motor are:

a) Induction machines are essentially constant speed machine.

b) The change of the speed with respect to the change of the load. As the torque of the load

increase, the speed of the motor will decrease gradually in the experiment. Hence, Torque

is inversely proportional to its speed (rpm).

c) The speed (rpm) of the motor decrease as the line current increase. Hence, line current

(A) of the source is also inversely proportional to its speed (rpm).

From the linearity of the two graphs obtained:

a) Operating the AC Motors at a very high torque is not efficient if the Speed of the motor is

reduced drastically.

b) Having no-load at initial start proves the frequency dependency on the input voltage

source.

c) AC motor does not require the complexity of having a load to start the motors. The speed

could be controlled by the source. Hence it is safer to start an AC motor.

d) AC motors will require lesser amount of current drawn than DC motors under load.

Hence it is suitable for industrial applications requiring heavy loads.

e) It will be easier to maintain the speed of the AC motors under load due to a lower current

drain from the source. From the graph, the current drop is less than 0.2A from loading of

torque from 0.1Nm to 1.0Nm.

12

CONCLUSION

In conclusion, with this experiment I have better understanding about the characteristics of DC

series and shunt motor and the AC three-phase motor.

For the series DC motor, if the load was very large for the motor size, the speed of the armature

would be very slow. If the load was light compared to the motor, the armature shaft speed would

be much faster, and if no load was present on the shaft, the motor could run away. The series

motor is capable of starting with a very large load attached, such as lifting applications

For the shunt DC motor, since the shunt field coil is made of fine wire, it cannot produce the

large current for starting like the series field. This means that the shunt motor has very low

starting torque, which requires that the shaft load be rather small. DC shunt motor can be easily

installed. The shunt motor is able to operate with rpm control while it is at high speed. Shunt

motor is generally used in belt-driven application.

The compound motor, a combination of the series motor and the shunt motor, is able to start with

fairly large loads and have some rpm control at higher speeds.

For AC Motor, based on the slip frequency equation , is small enough to neglect

and the AC motor behavior is much similar to the DC shunt motor. AC motors are ideal for most

industrial and commercial applications.

All in all, the objectives mentioned above have been met and we have also obtained

experimental proof by plotting out the curves, of the relationships between speed, torque &

current in DC motors and AC motor. Meanwhile, we have also understand from the graphs the

effect of torque and current have on the speed of the motor due to due kinds of field connections.

This has enabled us to choose more wisely for what motor is to be used for what purpose based

on the characteristics of the motor. We could have repeated the experiments for a more accurate

result.